Shaping of sheet metal has been accomplished by applying pressure to the central portion of a sheet metal work piece while the work piece is overlying a backing disc and secured by a peripheral clamp ring so as to cause the central portion of the work piece to distend outwardly into a dome-shaped product. The operation has been repeated in stages, as shown in U.S. Pat. No. 3,285,045, in order to increase the depth of the dome. A material limitation in this method, or the apparatus used to perform this method, has been the work hardening of the narrow zone adjacent the secured margins, or zones if the shaping is done in stages, with the attendant need of annealing the work piece between operations.
Attempts have been made to permit some slippage of the clamped margin, which tends to increase the area subject to work hardening and to reduce the amount of work hardening per unit area. However, control over such slippage has been too erratic to produce favorable results.